Large quantities of fly ash carried by the combustion products of power plants burning pulverized coal exist throughout the country and more is being created by operation of these plants. This accumulation creates a disposal problem and represents a waste of metal values, particularly aluminum, as a typical fly ash contains up to fourteen percent aluminum by weight. Lesser amounts of iron, titranium and other useful metals are present in fly ash.
No satisfactory process exists for economically recovering aluminum from fly ash having the required purity for commercial sale because of the difficulty of separating it from other metals present in the fly ash, particularly, iron. Separation through the chlorination route to recover aluminum as aluminum chloride looks attractive, however, the process must produce an aluminum chloride of substantial purity. For example, purity requirements for aluminum chloride feed material to an Alcoa-type aluminum cell limit the Fe.sub.2 O.sub.3 content of the feed to 0.03 percent. Furthermore, in the chlorination process, the chlorination of unwanted metals, such as silicon, must be suppressed to restrict the consumption of chlorine; otherwise, the process becomes prohibitively expensive.
A further problem involved in recovering the metal values from fly ash through the chlorination route, is the disposal of alkali and alkaline earth metal chlorides remaining in the final residue.
Accordingly, it is a principal object of this invention to provide a method for recovering aluminum of substantially high purity from fly ash and other materials containing iron and silica with the aluminum.
It is another object of this invention to provide a method for suppressing the chlorination of silicon when recovering aluminum as aluminum chloride from fly ash by chlorination.
It is a further object of this invention to provide a method for the disposal of alkali and alkaline earth metal chlorides remaining in the final residue resulting from the chlorination of fly ash to recover aluminum as aluminum chloride.